Bradley Manning's father has spoken out for the first time to denounce his son's treatment. Photograph: AP

Bradley Manning's father has broken his silence to denounce the way the Pentagon is treating the US soldier suspected of being the source of the WikiLeaks embassy cables.

Brian Manning said he had decided to speak out for the first time since his son was transferred to the military jail in Quantico marine base in Virginia, where he awaits court martial on charges of handing state secrets to an unauthorised party. Manning's father said he was moved into protesting because of the conditions under which his son is being detained.

"It's shocking enough that I would come out of our silence as a family and say, 'No, you've crossed a line. This is wrong'," Manning said.

He pointed out that his son had "not gone to trial or been convicted of anything." And he referred to the Guantánamo detention facility for terror suspects, saying: "They worry about people down in a base in Cuba, but here we have someone on our own soil under our own control, and they are treating him in this way".

Brian Manning said that the decision to strip the prisoner naked at night was a form of humiliation. The Pentagon has now said that it allows Bradley Manning to wear a garment at night, which his lawyer described as a smock.

Brian Manning made his remarks to the television programme Frontline. In the interview, he said that it was his encouragement that led to Bradley entering the army in the first place.

Manning's father had been in the navy as an intelligence specialist, which was how he met Bradley's mother, Susan, when he was stationed at a US base near Haverfordwest in Wales. The family lived in Crescent, a small town in Oklahoma where Bradley was born on 17 December 1987.

Brian Manning said his son joined the army "after I twisted his arm. He didn't want to join. But he needed structure in his life, he was aimless. I knew in my own life that joining the navy was the only thing that gave me structure, and everything's been fine since then."

He defended his son over allegations that he was the source of the secret documents taken from US military databases and passed to WikiLeaks. "I don't know why he would do that, I really don't." But he added that "whoever released those documents, it was the wrong thing to do. You just don't go there."